The ovaries are small, paired reproductive glands located on either side of the uterus within the pelvic cavity. They release an egg each month for potential fertilization and produce the hormones estrogen and progesterone, which regulate the menstrual cycle. For women in their reproductive years, average ovary volume is generally between 3 to 10 cubic centimeters (cc). It is common for the two ovaries to exhibit a slight difference in size, and this asymmetry is often a normal finding that is not a cause for concern.
Expected Variations Due to Anatomy and Cycle
A minor degree of anatomical asymmetry is standard for internal organs like the ovaries. Even when inactive, one ovary may be naturally somewhat larger than the other. In some cases, one ovary may be up to twice the volume of the other without indicating an underlying health issue.
The most frequent reason for a temporary size difference is the hormonal activity of the menstrual cycle. Each month, follicular development occurs in preparation for ovulation, usually favoring one ovary. This dominant follicle swells significantly as it fills with fluid, potentially reaching a diameter of up to 24 millimeters just before releasing the egg.
The ovary containing this dominant structure will appear temporarily enlarged on imaging compared to the resting ovary. After the egg is released, the remaining follicular structure collapses and transforms into the corpus luteum. This corpus luteum continues to produce hormones, which can maintain a temporary size disparity until it naturally degenerates into scar tissue, returning the ovary to its baseline size.
Common Medical Reasons for Significant Size Differences
While cyclical changes account for many temporary size differences, a persistent or medically significant size disparity often points toward a structural cause. The most common structural reason is the presence of an ovarian cyst, which is a fluid-filled sac that develops within the ovary.
Functional cysts, such as follicular or corpus luteum cysts, are benign and related to the normal cycle, typically resolving on their own within one to three months. Pathological cysts cause lasting enlargement because they do not spontaneously regress. Examples include endometriomas, which are cysts filled with old blood associated with endometriosis, or dermoid cysts, which contain various tissues like hair and fat.
Another common cause of persistent enlargement is Polycystic Ovary Syndrome (PCOS). While PCOS typically affects both ovaries, causing them to be enlarged and contain numerous small follicles, the degree of enlargement can sometimes be asymmetric. This asymmetric presentation can contribute to a noticeable size difference between the two ovaries. Rarely, a significant size difference can be caused by benign ovarian tumors, such as fibromas, or ovarian cancer, which usually presents as a complex mass that causes one ovary to become larger.
When to Seek Medical Guidance and What Monitoring Involves
If a medical professional detects an ovarian size difference, the assessment is typically performed using a pelvic ultrasound, which provides precise measurements of ovarian volume and structure. A size difference significantly greater than the normal two-fold variation, especially if associated with a volume exceeding 10 cubic centimeters, warrants further evaluation. The goal is to distinguish between a benign, functional cause and a condition requiring intervention.
Immediate medical consultation is warranted if the size difference is accompanied by acute symptoms. These warning signs include sudden, severe pelvic pain, which may indicate a cyst rupture or ovarian torsion, where the ovary twists on its blood supply. Patients should also report persistent abdominal bloating or fullness, unexpected weight loss, or any abnormal changes to the menstrual cycle or bleeding patterns.
For many functional or simple cysts, the standard medical approach involves watchful waiting. The patient may return for a repeat ultrasound after six to twelve weeks to confirm that the enlarged structure has resolved or decreased in size. If the enlargement persists or is caused by a pathological cyst, monitoring may continue, or a physician may discuss options for medical or surgical management depending on the size, symptoms, and potential risks.